EP 0 321 809, EP 0 780 629 or WO 93/17279 in each case disclose premix burners for operating with gaseous and/or liquid fuels, these burners having essential features in common. Thus, in each case, a swirl generator having tangential air inflow orifices encloses a cavity, the cross-sectional area of which widens in the axial flow direction. In EP 0 321 809 and EP 0 780 629, this is implemented by the swirl generator being of conical design, whilst the fully equivalent solution proposed in WO 93/17279 is to make the swirl generator itself cylindrical and insert inside the cavity a conical displacement body narrowing in the axial throughflow direction. Fuel is supplied to the swirl flow within the swirl generator. It is known for means for supplying a liquid fuel to be arranged in the vicinity of the burner axis and for means for introducing gaseous fuels to be provided radially on the outside, preferably in the region of the tangential air inflow orifices. The introduction of the fuels into a highly swirled flow is aimed at good premixing of the fuel/air mixture, and, of course, the axial component of the flow velocities must be so high that the flame does not flash back into the cavity of the burner. For further intensifying the intermixing of fuel and air, EP 0 780 629 proposes that the swirl generator be followed by a mixing section and that the swirl flow be transferred into this mixing section, if possible, without any loss. At a downstream end, the burner types mentioned issue with a more or less sudden widening of the flow cross section, at a short axial distance, in a combustion space. The highly swirled flow bursts open at this sudden jumping cross section, and a backflow bubble is formed, which causes a flame to be stabilized, without mechanical flame holders which are at risk from latent heat.
Burners of the type known from EP 0 321 809 have proved appropriate for many years in practical applications in gas turbines and atmospheric firing installations. The burners known from EP 0 321 809 and from EP 0 780 629 have undergone constant further development, and improvement proposals are found in a multiplicity of published documents.
However, another result arising from the functioning of the burners is that they build up high thermal stresses during operation. Thus, a burner of this type has a front plate, on which the swirl generator and, if appropriate, a mixing tube are mounted. In this case, the front plate constitutes the closure of the burner to the combustion space and separates from the combustion space a space from which air flows through the tangential orifices into the interior of the burner. In this case, both a leakage of combustion air and an uncontrolled penetration of smoke gases into the fresh air are to be avoided under all circumstances. Moreover, the entire burner has to be anchored in some way to the combustion space wall. Consequently, in the burners known at the present time, the swirl generator or, if present, a mixing tube is connected fixedly to the front plate, for example by welding. The front plate is then subjected, during operation, to hot combustion gases, whilst the rest of the structure is surrounded by a medium having a markedly lower temperature. The swirl generator and the mixing tube impede the free thermal expansion of the front plate, and high mechanical stresses are induced, precisely at the connection point which for manufacturing reasons is often a weld seam.
In modern gas turbines, the temperatures of the combustion air reach an order of magnitude of around 500.degree. C. However, with a rising pressure ratio of the working process or with pronounced external preheating of the combustion air, both of these being measures having a beneficial influence on the process efficiency, efficient cooling of the front plate is obviously more complicated to carry out.
It remains to be said, in conclusion, that, with a further rise in key process data, a limitation of the useful life of premix burners must be expected in the embodiments known at the present time, specifically, on the one hand, because of thermal stresses in the components due to impeded thermal expansion and, on the other hand, because the cooling of components exposed to hot gas will be always more complicated to achieve satisfactorily.